1. Statement of the Technical Field
The inventive arrangements concern RF amplifiers, and more particularly, RF power amplifiers which can be rapidly cycled between a transmit and standby state.
2. Description of the Related Art
FET type RF amplifiers are well known in the art. It is also well known that an FET type RF amplifier can be turned on and off by selectively controlling the application of a bias voltage to a gate of the FET RF transistor which comprises the amplifier. Depending on its value, the gate voltage can be used to either turn the FET off, or bias the FET into its linear operating region. A simple transistor switching circuit is conventionally used to turn the bias voltage on and off. Switching the bias voltage on or off in this way will control the FET RF amplifier transistor, turning it on and off as desired. Notwithstanding the simplicity of such circuits, they can have several limitations, particularly when controlling high power RF amplifier circuits which need to satisfy stringent distortion specifications.
For example, in order to reduce intermodulation (IM) distortion, it is sometimes desirable to connect a relatively large value of capacitor to the gate of the FET RF transistor. This capacitor can, depending on the specified requirements for IM distortion, have a value on the order of 10 μF. Although such a capacitor is useful for reducing IM distortion, it also creates certain problems. For example, the large value capacitor can cause these FET type RF amplifiers to require a relatively long time to transition between a fully on state, in which amplified RF energy is being output from the amplifier, to a fully off state, when the RF energy output is substantially terminated.
Quickly transitioning an RF power amplifier between an on and an off state can be important in certain applications. For example, certain types of networking and communications systems can require a transmitter to transmit only during certain limited and well defined time periods. For example, such systems can include TDMA based systems in which different transceiver units may be assigned certain time slots for transmitting data. In such systems RF amplifiers must be able to terminate RF transmissions at predetermined times. For example, it can be necessary in some applications to transition the RF amplifier between a fully on and fully off state in less than 40 μS. Accordingly, it is desirable to avoid any substantial delay which might occur between the application of a control signal which turns the RF amplifier on or off, and the actual time when RF energy output from the FET transistor finally terminates.
Another important concern when biasing FET RF amplifier circuits is stability. It can be desirable to maintain a very stable bias voltage over a large range of temperatures. Although stable voltage references can be used to help overcome this stability issue, the problem becomes considerably more complex in those instances where there is a need for dynamic control of the bias voltage. Conventional bias circuits with variable bias output settings can have an undesirable tendency to vary the bias voltage as a function of temperature.